We proposed to further define the factors controlling arterial PO2 in acute atelectasis. To do so, we will study the mechanisms regulating pulmonary blood flow to acutely atelectatic and hypoxic lobes of the lung. Hypoxia pulmonary vasoconstriction normally diverts flow away from these areas, but this effect is sometimes antagonized by unknown factors. We have shown that systemic hypoxemia is one such antagonizing factor. Distensibility of large pulmonary arteries is reduced in systemic hypoxemia, but the role this may play in regulating blood flow in atelectasis is unknown. We hypothesize that large vessel distensibility under the influence of the sympathetic nervous system interacts with hypoxic pulmonary vasoconstriction, by influencing the pulse pressure at the pre-capillary level, so that the two factors together determine net flow through atelectatic lung. We will perfuse the main pulmonary artery with a non-pulsatile flow from a reservoir, and with a pulsatile flow from a piston type pump. While measuring blood flow to the collapsed left lung with an electromagnetic flowmeter, systemic hypoxemia will be produced by changing from oxygen to room air ventilation. If the perfusion of the collapsed lung depends on systemic PO2 with pulsatile pump perfusion, as it does when perfused by the right ventricle, but this dependence is abolished by non-pulsatile perfusion, then a role for arterial distensibility changes will be indicated. If pharmacologic sympathetic blockade also abolished this dependence in pulsatile perfusion, further evidence of sympathetically-mediated distensibility changes will be gained. The influence of alveolar hypoxia and systemic hypoxemia on pulmonary capillary flow pulsatility will also be examined. Pulmonary capillary flow pulsatility will be measured by the N2O uptake technique in an hypoxic lung while systemic hypoxemia is prevented by ventilating the contralateral lung with oxygen. Capillary pulsatility will also be measured in a normoxic lung while system hypoxemia is produced by collapsing the contralateral lung.